The present invention generally relates to sol-gel glasses containing a relatively low concentration of metal centers, which glass has unique sensor and catalytic properties.
The application of the sol-gel process to the synthesis of dense oxide glasses and ceramics has been the subject of escalating interest because of the low fabrication temperatures and compositional flexibility provided by these materials. The sol-gel process utilizes the condensation reactions of silicon and/or metal alkoxides to provide a low temperature, synthetically flexible route to optical quality glasses.
The overall reaction for the synthesis of a silica sol-gel glass is:
Si(OR1)4+2H2Oxe2x86x92SiO2+4R1OH;
wherein tetramethylorthosilicate (TMOS) and tetraethylorthosilicate (TEOS) are the most common silicon alkoxides used. The ultimate production of a glassy material by this route, however, is accomplished in four primary stages: gelation, aging, drying/stabilization, and densification. The general process for forming sol-gels is taught in C. J. Brinker and G. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing (Academic Press, Boston, 1990) which is incorporated herein by reference.
The gelation step is composed of a series of condensation reactions in which silicon alkoxide condenses in the presence of water to form a Sixe2x80x94Oxe2x80x94Si bonded network and alcohol by-products. The xe2x80x9cgelxe2x80x9d that forms from the condensation reactions is soft and contains incompletely reacted material (remaining SiOH and SiOR1 functional groups), trapped organics (R1OH) and water. Aging of this gel (xcx9c3 months) results in further condensation to form Sixe2x80x94Oxe2x80x94Si linkages (which is accompanied by a large amount of shrinkage) and the evaporation of water and R1OH. The aged gels are dried (xcx9c100xc2x0 C.) and stabilized (xcx9c500xc2x0 C.) to remove any remaining organic material and to complete the condensation process.
The material remaining after stabilization, known as xe2x80x9cxerogel,xe2x80x9d has the properties of a hard, transparent glass, not unlike conventional fused silica glass. However, unlike fused silica glass, xerogel is porous with surface areas on the order of 200 m2/g. This pore structure can ultimately be closed at temperatures above 1000xc2x0 C. to yield a glass that is indistinguishable from fused silica glass.
The undensified silica xerogel material is unique because of its porous nature. Silica xerogels, acting as a host matrix, can be used to entrap and stabilize reactive chemical species while at the same time permitting small molecules to diffuse into the silica matrix and react. This imparts the reactivity of a particular functional group to an otherwise inert matrix.
Many of the materials made previously by the co-condensation of transition metals and silicon alkoxides have utilized relatively high metal concentrations and are often sintered into dense glasses. TiO2/silica sol-gels have been described extensively because of their importance as low thermal expansion glasses. Similarly, a good deal of work has also been reported on ZrO2 silicas. Recently, the properties of several SiO2/V2O5 materials have also been reported, including the bulk properties of densified glasses. A. Ghosh, et al., Appl. Phys. Lett., 59:855 (1991); N. Tohge, et al., J. Non-Cryst. Solids, 63:95 (1984).
Baiker, et al. have described the selective catalytic reduction of nitric oxide with stabilized SiO2/V2O5 xerogels that showed the highest catalytic activities at V5+ concentrations between 1 and 10%. A. Baiker, et al., J. Catal., 111:273 (1988).
The present invention relates to silica sol-gel compositions comprising relatively low concentrations of transition metals and uses thereof.
The present invention relates to a single phase metal-silica sol-gel glass formed by the co-condensation of a transition metal with silicon atoms where the metal atoms are uniformly distributed within the sol-gel glass as individual metal centers. Any transition metal may be used in the sol-gel glasses.
The present invention also relates to sensor materials where the sensor material is formed using the single phase metal-silica sol-gel glasses. The sensor materials may be in the form of a thin film or may be attached to an optical fiber. The present invention also relates to a method of sensing chemicals using the chemical sensors by monitoring the chromatic change of the metal-silica sol-gel glass when the chemical binds to the sensor.
The present invention also relates to thermal and photochemical oxidation catalysts where a metal-silica sol-gel glass catalyzes the reaction. The present invention also relates to a method of performing oxidation reactions using the metal-silica sol-gel glasses.
The present invention also relates to organopolymer metal-silica sol-gel composites where the pores of the metal-silica sol-gel glasses are filled with an organic polymer polymerized by the sol-gel glass.